Directionally solidified permanent magnet alloys with aligned ferro-magnetic whiskers



March 25. 1969 G. HEIMKE 3,434,892

DIRECTIONALLY SOLIDIFIED PERMANENT MAGNET ALLOYS WITH ALIGNED FERROMAGNETIC WHISKERS Filed Oct. 21, 1965 Sheet of 2 March 25, 1969 G. HEIMKE 3,434,892

DIRECTIONALLY SOLIDIFIED PERMANENT MAGNET ALLOYS WITH ALIGNED FERROMAGNETIC wmsxms Filed Oct. 21, 1965 Sheet 2 of 2 United States Patent U.S. Cl. 14831.57 5 Claims ABSTRACT OF THE DISCLOSURE A permanent magnetic material having the overall composition of a substantially eutectic alloy of at least one ferromagnetic metal with at least one non-magnetic element of the group consisting of aluminum, antimony, beryllium, boron, silicon, sulfur, tantalum, tin, titanium, and zirconium, at least part of said ferro-magnetic element being present in the for-m of aligned whiskers having a width of at least 50 and not more than 800 Angstrom and a length at least 1.2 times said width. The magnet material is produced by directional solidification through a temperature gradient in the range of 0.5 to 20 C./cm., and preferably 2 to 15 C./cm. Iron, cobalt, nickel, and alloys thereof are the ferro-magnetic components of the alloy.

This invention relates to metallic permanent magnets and to a method of preparing such magnets.

As the result of theoretical consideration, it has been recognized for some time that shape-anisotropic particles of ferromagnetic materials should have high coercive forces if their smallest dimensions in two directions perpendicularly to each other are of the order of magnitude of the thickness of the so-called Bloch wall and if their dimension in the third direction perpendicularly to the other two directions is a multiple of said smallest dimensions.

Attempts to prepare such fine magnetic particles by electrolytic precipitation of ferromagnetic metals on a mercury cathode did not produce coercive field strengths of the desired magnitude. The reason therefor is the ballchain like shape of said particles which, on the one hand, causes magnetization reversal to start at lower field strengths, and, on the other hand, reduces the obtainable packing densities by the stray fields generated during magnetization reversal. At higher packing densities, i.e. at small particle distances, said stray fields cause the particles to influence each other whereby a particle of lowest coercive field strength causes its higher coercive neighbors to magnetization reversal at said lower field strength. However, high packing densities of the ferromagnetic particles are necessary to produce higher remanence values.

Considerably :higher values of the coercive field strength could be observed in iron and iron-cobalt whiskers. There, the magnetization reversal takes place in such a way that the magnetization remains short-circuited always inside the individual particles; therefore, considerably higher packing densities can be employed for such whiskers, and accordingly higher remanences are available. The whiskers which had been used for these measurements, had been prepared in a high vacuum from the vapor phase by sublimation.

The particles obtained by electrodeposition on a mercury cathode must be aligned by application of a magnetic field in order to obtain favorable permanent magnetic properties. The same would apply to the vacuum-produced 3,434,892 Patented Mar. 25, 1969 whiskers if they are to be compacted to permanent magnets.

As the sublimation in a high vacuum is not a cornmercially attractive process for an annual production of several thousand tons, the problem remains to produce ferromagnetic whiskers of suitable shape in sufficiently dense packing and good orientation. Metal particles of such dimensions are strongly pyrophoric in air; therefore, their manipulation, e.g. for aligning in a magnetic field, requires additional protective procedures.

It is, therefore, a principal object of the invention to provide a permanent magnetic material containing oriented densely packed magnetic whiskers presenting superior magnetic properties.

Itis a further object of the invention to provide a convenient method for the preparation of such magnets.

Other objects and advantages will become apparent from the consideration of the specification and claims.

In accordance with the invention, the novel permanent magnetic material has an overall composition of a substantially eutectic alloy of at least one ferromagnetic metal with at least one non-magnetic element in which at least about 30 percent of the ferromagnetic metal is present in the form of oriented aligned whiskers having a transverse diameter in the range of about 50 to 800, preferably 100 to 300 A., embedded in a matrix of an alloy which is non-magnetic or very weakly magnetic. The length of the whiskers must be at least 1.2 times their diameter but, as a rule, will be a multiple of said value.

The ferromagnetic component of such alloys may be iron, cobalt, nickel or mixtures thereof; when nickel is used, it is of advantage to use it together with iron or cobalt, or iron-cobalt, whereby up to percent of said allow components may be replaced by nickel.

T-he non-magnetic component may be aluminum, antimony, beryllium, boron, silicon, sulfur, tantalum, tin, titanium, zirconium, or any combination of these elements.

In said eutectic or substantially eutectic alloys or mixtures, the whisker formation is obtained by slow cooling of the melt, whereby the temperature gradient in the solidification zoneand adjoining melt must always be parallel to the later direction of use of the permanent magnet.

The rate of cooling and the temperature gradient required for the production of the whiskers differ for the various alloys and is readily determined by preliminary tests.

The invention will now be described more in detail with reference to the accompanying drawings in which FIGS. 1 and 2 are cross-sectional views of apparatus suitable for the preparation of the novel permanent magnetic material.

Refer-ring first to FIG. 1 of the drawings, the reference numeral 1 designates an electrical furnace containing heating elements (not shown) and a control thermoelement 2. The heating zone is enclosed by a vertical ceramic refractory tube 3 closed on top by a ceramic refractory stopper 10 carrying two control thermoelements 8 and 9. From below, a ceramic tube 4 enters tube 3 covered by a refractory plate 5 supporting the crucible 6 in which the alloy is molten. 11 is an adjustable metal rod or cylinder which may be hollow for passage of a cooling fluid and which is used for the adjusted rate of cooling of the melt in the crucible.

In operation, the furnace is heated until the charge 7 in crucible 6 is molten and the melt has a temperature of 10 to C., preferably about 50 C. over the eutectic or liquidus temperatures. Then the cooling rod 11 is approached the ceramic plate 5 so as to apply to the melt in the crucible a rate of cooling and a temperature gradient found most suitable for the production of whiskers in the particular molten alloy. Said temperature gradient is controlled by the thermo-elements 8 and 9. After a stationary temperature distribution in the crucible zone has been established, the furnace is slowly, e.g. with suitable programming, cooled down until the entire melt has solidified.

Tube 4 may be stationary or adjustable. In the former case, it is generally necessary to bring the cooling rod in contact with the supporting plate or at least to move it so close thereto as to leave only a distance of a few, e.-g. 3 mm. In the latter case, the tube is supported for slow downward displacement together with rod 11 to remove the crucible 6 with the melt 7 out of the high temperature zone at a certain rate, thereby producing in the melt a temperature gradient whose steepness, at a given temperature distribution in the furnace, depends only on the rate of descent. As said rate of descent does not alloy to control separately the rate of cooling and the temperature drop, the cooling rod 11 is made separately adjustable with respect to tube 4 so that the temperature drop in the crucible can be additionally influenced by the spacing of the rod 11 from the supporting plate 5. For best results, the temperature difference between thermoelements 8 and 9 during the cooling period should be kept as constant as possible.

A somewhat different type of furnace is shown in FIG. 2. There, no crucible is used and the melting zone is defined by a ceramic tube 28 supported on a cooling tube 30 equipped for passage of a cooling fluid. The bottom of the melting zone is defined by a plate 29 supported on adjustable tube 31 which encloses an adjustable cooling rod or tube 32. When the alloy has been melted, the cooling element 32 is slowly brought close to the support 29 until the lower layers of the melt are cooled below the solidification temperature. Then the whole assembly 29, 31, 32 is lowered so as to slowly withdraw the solidified part of the alloy downwardly out of the melting zone. The rate of descent is adjusted by means of the thermoelement 9 in such a way that the solidification temperature remains at the same level. On further lowering of the tube 31 and cooling rod 32, the heat flux, which maintains the temperature gradient, passes from the melt through the solidified formed magnet rod to the cooling jacket 30. An inlet tube 27 is provided for feeding additional alloy composition to be melted. The other parts are designated by the same reference numerals as used in FIG. 1 and have the same function.

In this oven, magnet rods up to 11 cm. length could be made. At room temperature, sections were cut out which served for making the magnetic measurements which are given in the following examples.

The following examples are given to illustrate the production of some magnetic materials in accordance with the invention.

EXAMPLE 1 A composition corresponding to the eutectic alloy of iron with 14 percent by weight of titanium was heated and melted in crucible 7 of the furnace of FIG. 1 at a temperature of 1390" C., i.e. C. above the eutectic temperature.

In one test, the cooling rod 11 was approached the crucible at a rate producing therein a temperature drop of 10 C./cm., and cooling the furnace was programmed to a rate of 6 C./h. to a temperature of 1300 C. The magnetic measurement at room temperature showed a coercive field strength H of 680 oersteds and a remanence B of 5600 gauss.

The test was repeated whereby, however, for the cooling operation the cooling rod 11 was raised to a distance of 8 cm. from the ceramic plate 5. Then, the whole assembly, crucible 6 with melt 7, tube 4, and cooling rod 11 were lowered by about 10 cm. at a rate of 1.2 cm./hour.

In this case, the magnetic measurement at room temperature showed H =640 oersted and B =480O gauss.

In both cases, Fe whiskers had formed in a TiFe matrix.

EXAMPLE 2 An eutectic alloy composition consisting of 8.3 percent by weight of beryllium and 91.7 percent by weightof iron was molten in the furnace of FIG. 1 at a temperature of 1210 C., i.e. 45 C. above the eutectic temperature, and cooled down to 1150 C. with a temperature gradient of 12 C./cm. at a rate of cooling of 75 C./h. The obtained magnet rod had at room temperature a coercive force H of 540 oersteds and a remanence B of 5100 gauss.

If the test was repeated with placement of the cooling rod 11 cm. below the supporting plate and lowering of the entire assembly at a rate of 0.9 cm./ hour, the values found at room temperature were H =490 oersted, B =4200 gauss.

EXAMPLE 3 An alloy consisting of 14.5 percent by weight of zirconium, balance iron, i.e. an alloy somewhat differing from the eutectic composition, was heated at 1405 C., i.e. about 30 C. above the liquidus temperature. The cooling rod 11 was brought to a distance of 10 cm. from the supporting plate 5, and the assembly plate 5, crucible 6, tube 4, and cooling rod 11 was lowered at a rate of 1.4 cm./hour. At room temperature, there were found H =505 oersted, B =5300 gauss.

EXAMPLE 4 An eutectic alloy of 3.8 percent by weight of boron with iron was molten in the oven of FIG. 2 and heated at 1200 C., i.e. 40 C. above the eutectic temperature. Then the cooling member 32 was brought closer to the melt until the thermoelement 26 indicated a temperature of 1135 C. while the thermoelement 25 measured a temperature which was 35 C. higher.

By lowering the tube 30 with the cooling member 32 at a rate of about 1.5 cm./h., the temperature measured at the thermoelement 26, could be kept substantially constant within about C.

When additional material was charged through tube 27, the heat supply had to be adjusted accordingly.

EXAMPLE 5 An alloy composed of 16 percent by weight of zirconium, balance iron, was heated in the furnace of FIG. 2 at 1390 C., i.e. 60 C. above the eutectic temperature. The cooling member 32, was approached to the melt until the control thermoelement 26 registered a temperature of 1320 C. Thermoelement 25 indicated a temperature of 1350 C. The rate of descent of tube 30 and cooling member 32 was 0.5 to 4 cm./h., while the temperature measured at thermoelement 26 was kept constant within about :15 C.

Magnet rods up to a length of 7 cm. made from this alloy had a H =560690 oersted, B =39004600 gauss.

EXAMPLE 6 An iron-cobalt alloy (50:50) with 4.5 percent by weight of boron, i.e. an alloy of almost eutectic composition, was heated in the furnace of FIG. 2 at 1200 C. The cooling member 32 was approached to the melt until the control thermoelement 26 registered a temperature of 1100" C., whereby thermoelement 25 indicated 1150 C. The downward movement of the tubecooling element assembly was carried out at a rate of 0.5 to 4 cm./ h. and the temperature registered at thermoelement 26 was kept within :10 C. Magnet rods up to a length of 10 cm. made from this alloy had a H =240390 oersted, and a B =5000-6400 gauss.

In the magnetic measurements reported hereinaboye, the direction of the magnetic fields applied to the samples was the same as the direction of the temperature gradient during solidification. In measurements made perpendicularly thereto, the values found for the coercive strength and the remanence were always about 25 percent of the reported values.

If the solidification of the alloys was too slow or the temperature gradient maintained on cooling was too small, the whiskers obtained were too large and had diameters up to several am. and lengths up to 20 am. Such alloys had coercive strengths of about 5 to 80 oersteds only.

The preparation method set forth hereinabove ensures that the whiskers are developed in parallel orientation and at once fixed in this position, whereby they are embedded in the other component of the eutectic alloy. Therefore, the problems of the magnetic field orientation of the individual whiskers and their pyrophoric properties do not appear.

Nonetheless, the presence of a magnetic field during solidification may be of advantage, particularly for alloys which have not exactly eutectic composition, which are enriched in the ferromagnetic component, and whose Curie temperature is above the eutectic temperature. Thereby, the magnetic field should have a direction parallelly to the temperature gradient. Suitable for this treatment are, for instance, the eutectic or substantially eutectic alloys in the systems cobalt-gold, cobalt-sulfur, and cobalt-selenium.

When the alloys contain readily oxidized components, they must be melted and cooled in a protective atmosphere or in vacuo. This applies particularly to the eutectic or substantially eutectic alloys in the systems aluminumcobalt, beryllium-cobalt, berryllium-iron-cobalt, titaniumcobalt, titanium-iron-cobalt, boron-cobalt, silicon-cobalt, sulfur-iron, antimony-cobalt, antimony-iron, antimonyiron-cobalt, silicon-cobalt, silicon-iron, silicon-iron-cobalt, tin-cobalt, tantalum-cobalt, tantalum-iron, tantalum-ironcobalt, zirconium-cobalt, zirconium-iron-cobalt, and for alloys in which the ferromagnetic components iron, and/ or iron-cobalt, are replaced up to 80 percent by nickel.

The magnets solidified from the melt can be subjected to an annealing treatment which in some cases improves the magnetic and/or mechanical properties. This applies particularly for the alloys which contain iron and cobalt where the magnetic properties of the ferromagnetic whiskers can be influenced by arrangement and rearrangement operations.

The materials obtained by the method of the invention can be disintegrated and, if desired, pulverized, and they can then be compressed to shaped bodies, whereby binders may be added. Prior to, and/or during the compression, a magnetic field can be applied to reestablish the parallel orientation of the preferred magnetic directions defined by the position of the whiskers in the individual particles of the magnetic material. Such shaped bodies may be reinforced by sintering, or by curing of the binder. By embedding into a flexible plastic, the disintegrated magnetic materials obtained according to the invention can be made into flexible permanent magnets.

The following is an illustrative but not limitative list of eutectic or substantially eutectic alloys which are suitable for the production of whiskers by the method set forth above. All percentages are given by weight.

5-15% aluminum, balance cobalt, 2-10% beryllium, balance c obalt, 7-20% beryllium, balance iron, 2-20% beryllium, balance iron-cobalt, 18-28% titanium, balance cobalt,

5-29% titanium, balance iron, 5-30% titanium, balance iron-cobalt, 1-8% boron, balance cobalt,

0.5-8% boron, balance iron,

O.5-8% boron, balance iron-cobalt, 1-32% sulfur, balance cobalt,

l-36% sulfur, balance iron,

1-36% sulfur, balance iron-cobalt, 10-60% antimony, balance cobalt, 36-55% antimony, balance iron, 10-60% antimony, balance iron-cobalt, 8-17% silicon, balance cobalt, 18-32% silicon, balance iron,

8-32% silicon, balance iron-cobalt, 5-48% tin, balance cobalt,

15-50% tantalum, balance cobalt, 3-6l% tantalum, balance iron-cobalt, 2-27% zirconium, balance cobalt, 6-45% zirconium, balance iron, 2-45 zirconium, balance iron-cobalt.

Whenever iron-cobalt is a component of the alloy, such component may contain up to 60% of cobalt. In all alloys, up to 80% of the iron, cobalt, or iron-cobalt can be replaced by nickel.

I claim:

1. A permanent magnetic material having the overall composition of a substantially eutectic alloy of at least one ferromagnetic metal selected from the group consisting of iron, cobalt, nickel, and alloys thereof with the provision that nickel is present if at all, in amounts up to 80% of the iron, cobalt, or iron-cobalt alloys, with at least one non-magnetic element of the group consisting of aluminum, antimony, beryllium, boron, silicon, sulfur, tantalum, tin, titanium, and zirconium, said alloy having been directionally solidified and at least part of said ferro-magnetic element having been solidified in the form. of aligned whiskers having a width of at least and not more than 800 Angstrom and a length at least 1.2 times said Width.

2. A permanent magnetic material as claimed in claim 1 wherein said ferromagnetic component is a member of the group consisting of iron, cobalt, and iron-cobalt alloys containing up to percent of cobalt.

3. A permanent magnetic material as claimed in claim 2 wherein up to 80 percent by weight of said ferromagnetic component is replaced by nickel.

4. A permanent magnet as in claim 1 wherein said directional solidification is carried out using a temperature gradient of 0.5 to 20 C./cm.

5. A permanent magnet as in claim 1 wherein said directional solidification is carried out using a temperature gradient of 2 to 15 C./ cm.

References Cited UNITED STATES PATENTS 3,124,452 3/1964 Kraft -l35 3,132,022 5/1964 Luborsky et al. 148-1.6 XR 3,194,656 7/1965 Vordahl 75---135 3,226,225 12/1965 Weiss et a1.

L. DEWAYNE RUTLEDGE, Primary Examiner. P. WEINSTEN, Assistant Examiner.

US. Cl. X.R. 

